Re: [Haskell-cafe] generating Maybe
On 7 Nov 2007, at 12:40 PM, Tim Newsham wrote: Data.Maybe has functions for processing Maybe's but nothing useful for creating maybe. I think the following would be a very useful addition, a guarded function: guarded :: (a - Bool) - (a - b) - a - Maybe b guarded p f x | p x = Just (f x) | otherwise = Nothing such a function in the std libs would make functions like unfoldr more attractive -- uses of foldr nearly always encapsulate this notion. guarded p f x = guard (p x) f x which I would expect looks even better in practice: [1..10] = unfoldr (\ n - guard (n = 10) return (n, n + 1)) 1 vs. [1..10] = unfoldr (guarded (= 10) (\ n - return (n, n + 1)) 1 For example. jcc ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] generating Maybe
On 11/8/07, Tim Newsham [EMAIL PROTECTED] wrote: Data.Maybe has functions for processing Maybe's but nothing useful for creating maybe. I think the following would be a very useful addition, a guarded function: guarded :: (a - Bool) - (a - b) - a - Maybe b guarded p f x | p x = Just (f x) | otherwise = Nothing such a function in the std libs would make functions like unfoldr more attractive -- uses of foldr nearly always encapsulate this notion. How about this variant: ensure :: (MonadPlus m) = (a - Bool) - a - m a ensure p x | p x = return x | otherwise = mzero Which as Jonathan points out, could also be written: ensure p x = guard (p x) return x Now we can define guarded p f x = ensure p x fmap f Stuart ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] generating Maybe
On Thu, 8 Nov 2007, Stuart Cook wrote: On 11/8/07, Tim Newsham [EMAIL PROTECTED] wrote: Data.Maybe has functions for processing Maybe's but nothing useful for creating maybe. I think the following would be a very useful addition, a guarded function: guarded :: (a - Bool) - (a - b) - a - Maybe b guarded p f x | p x = Just (f x) | otherwise = Nothing such a function in the std libs would make functions like unfoldr more attractive -- uses of foldr nearly always encapsulate this notion. How about this variant: ensure :: (MonadPlus m) = (a - Bool) - a - m a ensure p x | p x = return x | otherwise = mzero Which as Jonathan points out, could also be written: ensure p x = guard (p x) return x Now we can define guarded p f x = ensure p x fmap f My 'toMaybe' is a specialisation of '\b x - guard b return x'. This let us return to the discussion, whether there should be a specialised function, if there is a general function which does the same. http://www.haskell.org/haskellwiki/Simple_to_complex#Type_class_methods I use 'toMaybe' a lot, because it allows partial application and I have seen library code, where 'toMaybe' could have been used, but its author didn't seem to know or like 'guard b return x'. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] generating Maybe
On Wed, 7 Nov 2007, Tim Newsham wrote: Data.Maybe has functions for processing Maybe's but nothing useful for creating maybe. I think the following would be a very useful addition, a guarded function: guarded :: (a - Bool) - (a - b) - a - Maybe b guarded p f x | p x = Just (f x) | otherwise = Nothing such a function in the std libs would make functions like unfoldr more attractive -- uses of foldr nearly always encapsulate this notion. This would be guarded p f x = toMaybe (p x) (f x) in terms of my beloved but already rejected 'toMaybe': http://www.haskell.org/pipermail/libraries/2004-July/002326.html ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
